1. Technical Field
The present disclosure relates to a copper conductor with an anodized aluminum dielectric layer and a method for making the same.
2. Background Art
The general idea of creating an electrically insulating coating layer on a conducting material is well known. For example, organic wire coatings of polyesters, polyimides, thermoset epoxies, silicone rubbers, and many others have been used in a variety of applications for many years. These types of materials have very good dielectric properties and are able to withstand high voltages. However, they typically are limited to applications with operating/environmental temperatures below about 200-220° C. and are not suitable for high current density or severe environment applications. In addition, polymeric coatings are excellent thermal insulators, which is undesirable for dissipation of ohmic or resistance heating in coil windings. Inorganic wire coverings or coatings, such as glass-fiber sheaths, glass encapsulation, mica, or ceramic materials, may be used to tolerate higher temperatures, but tend to be relatively thick, brittle, and have low radial dimensional control so that they are not amenable to forming processes common in manufacturing electrical machines.
Anodizing electrically conductive materials such as aluminum or copper has been done for nearly a century. Many overhead transmission lines are implemented by aluminum conductors with a thin (about 1 micron) outer layer of aluminum oxide formed by anodization to resist corrosion. However, this layer or skin is too thin to electrically insulate the conductor, so that other measures are required. While suitable for some overhead transmission line applications, the bulk resistance of aluminum wire is generally too high for electromagnetic coil and electrical machine applications.
Copper is generally preferred for conductors used in electromagnetic machines due to its high electrical conductivity. Electroplating aluminum on copper has been attempted, but the aluminum tends to oxidize before it chemically attaches to the copper so that a poor bond is formed and the aluminum layer flakes off of the copper core. Copper can be plated onto an aluminum conductor core, but does not provide the desired electrical characteristics as described above. Copper can also be anodized as disclosed in U.S. Pat. Nos. 5,078,844 and 5,401,382. However, the direct anodization of copper as described in these patents is subject to high strain and cracking as shown by the dielectric strength drop described in U.S. Pat. No. 5,501,382, and the coatings of copper are porous, which makes it difficult or impossible to halt the oxidation process, eventually resulting in an electrical short or breakdown of the wire.
An electrically insulated wire having a copper or copper alloy core conductor with an aluminum oxide layer used to improve adhesion between the conductor core and an outer oxide film insulating layer is disclosed in U.S. Pat. No. 5,091,609. As described in the '609 patent, a thin aluminum or aluminum alloy layer is anodized to form an anodic oxide film having a thickness of only about 10-15 microns thick, which is porous and has a large number of holes passing from its surface toward the base material so that it is generally impossible to obtain an insulating strength which is proportional to the film thickness of the oxide film. This problem is solved using a sol-gel process or acid salt pyrolytic process to fill the holes with an additional oxide insulating layer having a smooth outer surface that decreases gas adsorption and provides electrical insulation proportional to the film thickness.